FILICES 83 



siderably greater divergence ; the affinity of Osmundaceae and Schizaeaceae 

 with the typical ferns is more remote ; while the Marattiaceae exhibit so 

 many points of divergence that some high authorities have removed 

 them altogether from the Filices. 



ORDER i. POLYPODIACE^E. 



This order includes by far the largest number of genera and species, 

 and may be regarded as the typical family of ferns. The sporanges 

 arise out of single epidermal cells, and have usually long pedicels ; they 



FIG. 57. Alsophila aculeata Klotzsch, a tree-fern (reduced). 



have an incomplete vertical annulus, and consequently dehisce trans- 

 versely. The sori vary greatly in size and form, and usually consist of 

 a large number of sporanges. They are seated on the under side of the 

 divided or undivided leaf, upon the veins ; much less often (Acrosticheae) 

 also on the upper surface or in connection with the mesophyll ; they 

 are usually covered by an indusium in the Asplenieae, Aspidiese, and 

 Davallieae, but not in the Acrosticheae or Polypodiese. In the great 

 majority of species all the leaves are ultimately fertile; but in some genera 

 (Gymnogramme (Desv.), Lomaria (Willd.), Platycerium (Desv.), &c.) 



G 2 



